Eli5: why is there no test for the “chemical imbalance” that is often mentioned for depression?
In: Chemistry
If it’s any similar to some other hormones like the HPA axis, it’s because there’s only a very low amount of that chemical circulating in your body at a time. Most of it is produced in your brain and used a very short distance ‘downstream’ from there, which effectively leads to undetectable amounts in the wider circulation. So if you took a blood sample from someone’s arm like we normally do, you would hardly detect the chemical, let alone be able to tell if there’s an imbalance. You’d have to take a blood sample inside the brain between the two specific points where the chemical is produced and absorbed again, to be able to tell anything at all.
That being said, things like depression or adhd are usually caused by neurotransmitter problems. They are molecules that you find in the brain and that allow one neurone to communicate with the next, but they are not found in blood much at all. So again you’d have to somehow sample the connection between two neurones to measure the amount of neurotransmitter.
An easier way to do it is to monitor the effect of drugs that affect neurotransmitters. If you give them SSRIs (serotonin reuptake inhibitors), what you are doing is forcing serotonin to be present for a longer time before it is reabsorbed, thereby making sure the signal travels correctly from one neurone to the next. Normal people will react differently to the drug compared to depressed people with a serotonin deficiency as it were. Compare it to to adhd meds: normal people on adderall act all high, while people with adhd on adderall are able to calm down
“Chemical imbalance” is a pretty inexact way of describing the theories for how depression is driven by body chemistry. But in any case, that chemistry is largely in the brain. The brain is separated pretty strongly from the rest of the body (only a few chemicals can move into or out of the brain into the bloodstream) meaning that it’s not something that shows up on a blood test.
It’s also not as simple as the amount of single chemicals: the brain depends on a mix of different chemicals, whose behavior depends on the concentrations of the others and whose effects depend not just on the chemicals present but on the receptors available on each cell.
Where exactly are you getting your facts from? People used to think diseases and viruses were a chemical imbalance, that’s how bloodletting became a thing..
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